1. Field of the Invention
The present invention relates to a nanoparticle, a method of producing a nanoparticle, and a magnetic recording medium.
2. Description of the Related Art
In order to increase magnetic recording density, it is necessary to decrease the particle size of magnetic bodies contained in a magnetic layer. In magnetic recording media used widely in videotapes, computer tapes, disks, and the like, noise decreases with the decrease in particle size when the mass of the ferromagnetic body is the same.
CuAu type or Cu3Au type hard magnetic regular alloys have large crystal magnetic anisotropy because of distortion caused when regulated so that they exhibit hard magnetic characteristics even if they are reduced in particle size and put in a nanoparticle state. Therefore, these alloys are promising materials for improving magnetic recording density.
Examples of methods for synthesizing nanoparticles capable of forming these CuAu type or Cu3Au type alloys when classified by precipitation method include (1) an alcohol reduction method using a primary alcohol; (2) a polyol reduction method using a secondary, tertiary, divalent or trivalent alcohol; (3) a heat decomposition method; (4) an ultrasonic decomposition method; and (5) a strong reducing agent reduction method.
Also, when classified by a reaction system, methods for synthesizing nanoparticles include (6) a polymer existence method; (7) a high-boiling point solvent method; (8) a regular micelle method; and (9) an reverse micelle method.
The alcohol reduction method (1) has poor reduction ability. Therefore, when reducing a precious metal and a base metal at the same time, it is hard to form a uniform alloy and many alloys end up having a core/shell structure. In the case of the polyol reduction method (2) and the heat decomposition method (3), a high-temperature reaction is required and these methods are therefore inferior in production aptitude. The ultrasonic decomposition method (4) and the strong reducing agent reduction method (5) are relatively simple methods. However, in these methods, coagulation and precipitation tend to be caused and it is therefore difficult to obtain a small monodispersible particle without implementing a special technique in the reaction system.
There is also an ethanol reduction method using polyvinylpyrrolidone, in which the above-mentioned methods (1) and (6) are combined. In this case, the amount of polymers after synthesis is very large and is difficult to decrease to the required amount.
For a system in which methods (2), (3) and (7) are combined, those described in Japanese Patent Application Laid-Open (JP-A) No. 2000-54012 and U.S. Pat. No. 6,254,662 are known. This method is, however, very hazardous because highly toxic substances are used. Also, in these methods, it is necessary to run a reaction in inert gas and at a temperature as high as nearly 300° C., hence these methods have the drawback that the apparatuses used are complicated and thus inferior from the standpoint of production aptitude.
Methods using a system combining methods (5) and (8) and a system combining methods (5) and (9) are common methods. However, detailed conditions and the like as to a method for obtaining metal nanoparticles having the intended composition and particle size have yet to be found.
The nanoparticles synthesized in the above methods have a face centered cubic crystal structure. The face-centered cubic crystal usually exhibits soft magnetism or paramagnetism These nanoparticles exhibiting soft magnetism or paramagnetism are not adaptable to recording media. In order to obtain a hard magnetic regular alloy having a coercive force of 95.5 kA/m (1200 Oe) or more, which is necessary for magnetic recording media, annealing treatment must be carried out at a temperature higher than the transformation temperature at which the alloy is transformed from an irregular phase to a regular phase.
However, when the nanoparticle produced in the above methods is applied to a support, followed by annealing treatment to produce a magnetic recording medium, these nanoparticles tend to coagulate easily with each other leading to reduced coatability and deteriorated magnetic characteristics. It is also difficult to form a perfect regular phase even if heat treatment is performed because the particle diameter of the resulting nanoparticle is uneven and therefore, there are cases where the desired hard magnetism is not obtained.
Also, the transformation temperature is generally as high as 500° C. or more and an organic support, which is commonly used, does not possess adequate heat resistance. It is therefore difficult to form a magnetic film by applying a nanoparticle to the organic support, followed by carrying out annealing treatment.